Merge branch 'assign-issues-for-merge-request-18876' into 'master'

Ability to bulk assign issues to author of merge request

## What does this MR do?
Provides a link to auto-assign issues to the author of a merge request, when they are mentioned as being closed by the MR.

## Are there points in the code the reviewer needs to double check?


## Why was this MR needed?
To help avoid working on a MR without having assigned related issues to self

## What are the relevant issue numbers?
Fixes #18876 

## Screenshots (if relevant)
![ScreenShot-P216](/uploads/1af5e71a0a0ff0a60c5d7b54c0e09d9c/ScreenShot-P216.png)

## Tasks
- [x] Refactor or move away from using `BulkUpdateService`
- [x] ~~Consider alternate link message when only a subset of issues will be assigned~~
- [x] Minimize repeated calls to expensive `closes_issues` method
- [x] Move away from using inflector for pluralization and fix flash message
- [x] Change auth `before_action` and fallback to error flash message
- [x] Shouldn't overwrite current assignee if one exists

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] ~~[Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)~~
- [x] ~~API support added~~
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5725

1 files changed, 35 insertions, 0 deletions

diff --git a/app/services/merge_requests/assign_issues_service.rb b/app/services/merge_requests/assign_issues_service.rb
new file mode 100644
index 00000000000..f636e5fec4f
--- /dev/null
+++ b/app/services/merge_requests/assign_issues_service.rb
@@ -0,0 +1,35 @@
+module MergeRequests
+  class AssignIssuesService < BaseService
+    def assignable_issues
+      @assignable_issues ||= begin
+        if current_user == merge_request.author
+          closes_issues.select do |issue|
+            !issue.assignee_id? && can?(current_user, :admin_issue, issue)
+          end
+        else
+          []
+        end
+      end
+    end
+
+    def execute
+      assignable_issues.each do |issue|
+        Issues::UpdateService.new(issue.project, current_user, assignee_id: current_user.id).execute(issue)
+      end
+
+      {
+        count: assignable_issues.count
+      }
+    end
+
+    private
+
+    def merge_request
+      params[:merge_request]
+    end
+
+    def closes_issues
+      @closes_issues ||= params[:closes_issues] || merge_request.closes_issues(current_user)
+    end
+  end
+end